Excited-state absorption measurement of Tm³⁺-doped crystals

Szela, J.W. and Mackenzie, J.I. (2012) Excited-state absorption measurement of Tm³⁺-doped crystals. SPIE Photonics Europe 2012, , Brussels, Belgium. 16 - 19 Apr 2012. (doi:10.1117/12.922673).

Abstract

The excited-state absorption (ESA) spectra from the long-lived ³F₄ energy-level of several crystals doped with trivalent thulium (Tm3+) ions have been measured using a pump-probe technique employing high-brightness narrowband (FWHM ~15 nm) light emitting diode (LED) as the probe. Our aim is to determine the strength of ESA channels at wavelengths addressable by commercially available laser diodes around 630-680 nm, pivotal for building waveguide upconversion lasers. The favorable lifetime of the ³F₄ energy-level and negligible ground-state absorption for the red-wavelength second-step excitation, ensures a direct and efficient excitation route for a dual-wavelength pumping scheme of the thulium ion, which will enable blue-green laser emission from its ¹G₄ manifold.
ESA measurements in the red spectral region were conducted at room temperature utilizing the pump-probe technique. The advantage of the LED probe over a broadband incandescent lamp is that stimulated-emission at the probe wavelength can be eliminated, simplifying the analysis of the measurement and the determination of the magnitude of the ESA cross-section. Four Tm-doped crystalline hosts Y₃Al₅O₁₂ (YAG), YAlO₃ (YAP), LiYF₄ (YLF), and KYGdLu(WO₄)₂ (KYGdLuW) were investigated, with polarized ESA spectra measured for the non-isotropic samples. Excitation of the Tm₃₊ ions into the intermediate ³F₄ manifold was achieved via non-radiative and cross-relaxation energy transfer from the ³H₄ manifold, pumped by a current-modulated fibre-coupled GaAs laser diode operating at ~790 nm. To ensure that ESA at all wavelengths of interest were resolved, the LED's spectra were tuned via the modulated drive-current amplitude and junction temperature. As such, with direct pump and probe modulation, we eliminated the need for mechanical choppers, simplifying the optimization of frequency-resolved ESA measurements needed to isolate the other possible ESA transition (³H₄ to ¹D₂). Vastly improved ESA spectra have been obtained with respect to those reported in the literature.

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Contributors

Author: J.I. Mackenzie

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